Magnetic tunnel junction magnetoresistive read head with sensing layer as rear flux guide

ABSTRACT

A magnetic tunnel junction (MTJ) magnetoresistive read head for a magnetic recording system has the MTJ sensing or free ferromagnetic layer also functioning as a flux guide to direct magnetic flux from the magnetic recording medium to the tunnel junction. The MTJ fixed ferromagnetic layer and the MTJ tunnel barrier layer have their front edges substantially coplanar with the sensing surface of the head. Both the fixed and free ferromagnetic layers are in contact with opposite surfaces of the MTJ tunnel barrier layer but the free ferromagnetic layer extends beyond the back edge of either the tunnel barrier layer or the fixed ferromagnetic layer, whichever back edge is closer to the sensing surface. This assures that the magnetic flux is non-zero in the tunnel junction region. The magnetization direction of the fixed ferromagnetic layer is fixed in a direction generally perpendicular to the sensing surface and thus to the magnetic recording medium, preferably by interfacial exchange coupling with an antiferromagnetic layer. The magnetization direction of the free ferromagnetic layer is aligned in a direction generally parallel to the surface of the medium in the absence of an applied magnetic field and is free to rotate in the presence of applied magnetic fields from the medium. A layer of high coercivity hard magnetic material adjacent the sides of the free ferromagnetic layer longitudinally biases the magnetization of the free ferromagnetic layer in the preferred direction.

RELATED APPLICATION

This application is related to concurrently filed application Ser. No.08/957,699 titled "MAGNETIC TUNNEL JUNCTION MAGNETORESISTIVE READ HEADWITH SENSING LAYER AS FLUX GUIDE", and application Ser. No. 08/957,787titled "SHIELDED MAGNETIC TUNNEL JUNCTION MAGNETORESISTIVE READ HEAD".

TECHNICAL FIELD

This invention relates in general to magnetic tunnel junction (MTJ)devices, and more particularly to an MTJ device for use as amagnetoresistive (MR) head for reading magnetically-recorded data.

BACKGROUND OF THE INVENTION

A magnetic tunnel junction (MTJ) device is comprised of twoferromagnetic layers separated by a thin insulating tunnel barrier layerand is based on the phenomenon of spin-polarized electron tunneling. Oneof the ferromagnetic layers has a higher saturation field in onedirection of an applied magnetic field, typically due to its highercoercivity than the other ferromagnetic layer. The insulating tunnelbarrier layer is thin enough that quantum mechanical tunneling occursbetween the ferromagnetic layers. The tunneling phenomenon iselectron-spin dependent, making the magnetic response of the MTJ afunction of the relative orientations and spin polarizations of the twoferromagnetic layers.

MTJ devices have been proposed primarily as memory cells for solid statememory. The state of the MTJ memory cell is determined by measuring theresistance of the MTJ when a sense current is passed perpendicularlythrough the MTJ from one ferromagnetic layer to the other. Theprobability of tunneling of charge carriers across the insulating tunnelbarrier layer depends on the relative alignment of the magnetic moments(magnetization directions) of the two ferromagnetic layers. Thetunneling current is spin polarized, which means that the electricalcurrent passing from one of the ferromagnetic layers, for example, alayer whose magnetic moment is fixed or prevented from rotation, ispredominantly composed of electrons of one spin type (spin up or spindown, depending on the orientation of the magnetic moment of theferromagnetic layer). The degree of spin polarization of the tunnelingcurrent is determined by the electronic band structure of the magneticmaterial comprising the ferromagnetic layer at the interface of theferromagnetic layer with the tunnel barrier layer. The firstferromagnetic layer thus acts as a spin filter. The probability oftunneling of the charge carriers depends on the availability ofelectronic states of the same spin polarization as the spin polarizationof the electrical current in the second ferromagnetic layer. Usually,when the magnetic moment of the second ferromagnetic layer is parallelto the magnetic moment of the first ferromagnetic layer, there are moreavailable electronic states than when the magnetic moment of the secondferromagnetic layer is aligned antiparallel to that of the firstferromagnetic layer. Thus, the tunneling probability of the chargecarriers is highest when the magnetic moments of both layers areparallel, and is lowest when the magnetic moments are antiparallel. Whenthe moments are arranged neither parallel nor antiparallel, thetunneling probability takes an intermediate value. Thus, the electricalresistance of the MTJ memory cell depends on the spin polarization ofthe electrical current and the electronic states in both of theferromagnetic layers. As a result, the two possible magnetizationdirections of the ferromagnetic layer whose magnetization direction isnot fixed uniquely define two possible bit states (0 or 1) of the memorycell.

A magnetoresistive (MR) sensor detects magnetic field signals throughthe resistance changes of a sensing element, fabricated of a magneticmaterial, as a function of the strength and direction of magnetic fluxbeing sensed by the sensing element. Conventional MR sensors, such asthose used as MR read heads for reading data in magnetic recording diskdrives, operate on the basis of the anisotropic magnetoresistive (AMR)effect of the bulk magnetic material, which is typically permalloy (Ni₈₁Fe₁₉). A component of the read element resistance varies as the squareof the cosine of the angle between the magnetization direction in theread element and the direction of sense current through the readelement. Recorded data can be read from a magnetic medium, such as thedisk in a disk drive, because the external magnetic field from therecorded magnetic medium (the signal field) causes a change in thedirection of magnetization in the read element, which in turn causes achange in resistance of the read element and a corresponding change inthe sensed current or voltage. In conventional MR read heads, incontrast to MTJ devices, the sense current is in a direction parallel tothe ferromagnetic layer of the read element.

The use of an MTJ device as a magnetoresistive read head for magneticrecording has also been proposed, as described in U.S. Pat. No.5,390,061. In this MTJ read head, the free and fixed ferromagneticlayers have lateral perimeters which do not extend beyond that of thelateral perimeter of the insulating tunnel barrier. In a magneticrecording device the read head senses flux from small magnetized regionsor magnetic bits written into a thin film magnetic medium above whichthe head is suspended. Increased capacity disk drives are achieved inpart by higher magnetic bit areal densities. Thus the area of eachmagnetic region or bit must be decreased but this thereby gives rise toreduced magnetic flux. Magnetic recording heads which can sense reducedmagnetic flux with greater output signal are thereby required for higherperformance and higher capacity magnetic recording disk drives. Improvedmagnetic recording read heads can be obtained by finding MTJ structureswith higher magnetoresistance coefficients. However the MR coefficientsare determined by the intrinsic electronic and magnetic properties ofthe materials comprising the MTJ.

What is needed is an MTJ MR read head for a magnetic recording systemwhich gives greater output signal for the same input magnetic flux forotherwise the same set of magnetic and electrical materials from thewhich MTJ is constructed.

SUMMARY OF THE INVENTION

The invention is an MTJ MR read head for a magnetic recording systemwherein the free ferromagnetic layer also acts as a rear flux guide todirect magnetic flux from the magnetic recording medium to the tunneljunction. In a magnetic recording disk drive embodiment, the fixedferromagnetic layer, the tunnel barrier layer and free ferromagneticlayer all have their edges exposed at the air-bearing surface (ABS).Both the fixed and free ferromagnetic layers are in contact withopposite surfaces of the tunnel barrier layer but the free ferromagneticlayer extends beyond the back edge of either the tunnel barrier layer orthe fixed ferromagnetic layer, whichever back edge is closer to thesensing surface. This assures that the magnetic flux is non-zero in thetunnel junction region. The magnetization direction of the fixedferromagnetic layer is fixed in a direction generally perpendicular tothe ABS and thus to the disk surface, preferably by interfacial exchangecoupling with an antiferromagnetic layer. The magnetization direction ofthe free ferromagnetic layer is aligned in a direction generallyparallel to the surface of the ABS in the absence of an applied magneticfield and is free to rotate in the presence of applied magnetic fieldsfrom the magnetic recording disk. A layer of high coercivity hardmagnetic material adjacent the sides of the free ferromagnetic layerlongitudinally biases the magnetization of the free ferromagnetic layerin the preferred direction.

The MTJ MR read head may be formed as part of an integrated read/writehead structure in which there are electrically conducting magneticshields located on both sides of the MTJ MR read head. The electricalleads for sensing circuitry are formed on the two shields so that anelectrical path is provided from the shields through the leads to thefixed and free ferromagnetic layers of the tunnel junction.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified block diagram of a conventional magneticrecording disk drive for use with the recessed MTJ MR read headaccording to the present invention.

FIG. 2 is a top view of the disk drive of FIG. 1 with the cover removed.

FIG. 3 is a vertical cross-section of a conventional integratedinductive write head/MR read head with the MR read head located betweenshields and adjacent to the inductive write head for the purpose ofillustrating where the MTJ MR read head of the present invention wouldbe located.

FIG. 4 is a cross-section view taken through the tunnel junction of theMTJ MR read head of the present invention and illustrates theperpendicular direction of current flow through the tunnel junction.

FIG. 5 is a sectional view of the rear flux guided MTJ MR read headillustrating the location of the various layers relative to sensing endof the head (the air-bearing surface).

FIGS. 6A-6N illustrate steps in the fabrication of the rear flux guidedMTJ MR read head of the present invention.

FIG. 7 is a view of the sensing surface illustrating the front edge ofthe MTJ device and the edges of the longitudinal biasing ferromagneticlayers.

DETAILED DESCRIPTION OF THE INVENTION

Prior Art

Referring first to FIG. 1, there is illustrated in sectional view aschematic of a prior art disk drive of the type using a MR sensor. Thedisk drive comprises a base 10 to which are secured a disk drive motor12 and an actuator 14, and a cover 11. The base 10 and cover 11 providea substantially sealed housing for the disk drive. Typically, there is agasket 13 located between base 10 and cover 11 and a small breather port(not shown) for equalizing pressure between the interior of the diskdrive and the outside environment. A magnetic recording disk 16 isconnected to drive motor 12 by means of hub 18 to which it is attachedfor rotation by the drive motor 12. A thin lubricant film 50 ismaintained on the surface of disk 16. A read/write head or transducer 25is formed on the trailing end of a carrier, such as an air-bearingslider 20. Transducer 25 is a read/write head comprising an inductivewrite head portion and a MR read head portion, as will be described withrespect to FIG. 3. The slider 20 is connected to the actuator 14 bymeans of a rigid arm 22 and a suspension 24. The suspension 24 providesa biasing force which urges the slider 20 onto the surface of therecording disk 16. During operation of the disk drive, the drive motor12 rotates the disk 16 at a constant speed, and the actuator 14, whichis typically a linear or rotary voice coil motor (VCM), moves the slider20 generally radially across the surface of the disk 16 so that theread/write head 25 may access different data tracks on disk 16.

FIG. 2 is a top view of the interior of the disk drive with the cover 11removed, and illustrates in better detail the suspension 24 whichprovides a force to the slider 20 to urge it toward the disk 16. Thesuspension may be a conventional type of suspension, such as thewell-known Watrous suspension, as described in IBM's U.S. Pat. No.4,167,765. This type of suspension also provides a gimbaled attachmentof the slider which allows the slider to pitch and roll as it rides onthe air bearing. The data detected from disk 16 by the transducer 25 isprocessed into a data readback signal by signal amplification andprocessing circuitry in the integrated circuit chip 15 located on arm22. The signals from transducer 25 travel via flex cable 17 to chip 15,which sends its output signals to the disk drive electronics (not shown)via cable 19.

FIG. 3 is a cross-sectional schematic view of the integrated read/writehead 25 which includes a MR read head portion and an inductive writehead portion. The head 25 is lapped to form an air-bearing surface(ABS), the ABS being spaced from the surface of the rotating disk 16(FIG. 1) by the air bearing as discussed above. The read head includes aMR sensor 40 sandwiched between first and second gap layers G1 and G2which are, in turn, sandwiched between first and second magnetic shieldlayers S1 and S2. In a conventional disk drive, the MR sensor 40 is anAMR sensor. The write head includes a coil layer C and insulation layerI2 which are sandwiched between insulation layers I1 and I3 which are,in turn, sandwiched between first and second pole pieces P1 and P2. Agap layer G3 is sandwiched between the first and second pole pieces P1,P2 at their pole tips adjacent to the ABS for providing a magnetic gap.During writing, signal current is conducted through the coil layer C andflux is induced into the first and second pole layers P1, P2 causingflux to fringe across the pole tips at the ABS. This flux magnetizescircular tracks on the rotating disk 16 during a write operation. Duringa read operation, magnetized regions on the rotating disk 16 inject fluxinto the MR sensor 40 of the read head, causing resistance changes inthe MR sensor 40. These resistance changes are detected by detectingvoltage changes across the MR sensor 40. The voltage changes areprocessed by the chip 15 (FIG. 2) and drive electronics and convertedinto user data. The combined head 25 shown in FIG. 3 is a "merged" headin which the second shield layer S2 of the read head is employed as afirst pole piece P1 for the write head. In a piggyback head (not shown),the second shield layer S2 and the first pole piece P1 are separatelayers.

The above description of a typical magnetic recording disk drive with anAMR read head, and the accompanying FIGS. 1-3, are for representationpurposes only. Disk drives may contain a large number of disks andactuators, and each actuator may support a number of sliders. Inaddition, instead of an air-bearing slider, the head carrier may be onewhich maintains the head in contact or near contact with the disk, suchas in liquid bearing and other contact and near-contact recording diskdrives.

PREFERRED EMBODIMENTS

The present invention is a MR read head with an MTJ sensor for use inplace of the MR sensor 40 in the read/write head 25 of FIG. 3.

FIG. 4 is a section view of the MTJ MR read head of the presentinvention as it would appear if taken through a plane whose edge isshown as line 42 in FIG. 3 and viewed from the disk surface. Thus thepaper of FIG. 4 is a plane parallel to the ABS and through substantiallythe active sensing region, i.e., the tunnel junction, of the MTJ MR readhead to reveal the layers that make up the head.

Referring to FIG. 4, the MTJ MR read head includes an electrical lead102 formed on the gap layer G1 substrate, an electrical lead 104 belowgap layer G2, and the MTJ 100 formed as a stack of layers betweenelectrical leads 102, 104.

The MTJ 100 includes a first electrode multilayer stack 110, aninsulating tunnel barrier layer 120, and a top electrode stack 130. Eachof the electrodes includes a ferromagnetic layer in direct contact withtunnel barrier layer 120, i.e., ferromagnetic layers 118 and 132.

The base electrode layer stack 110 formed on electrical lead 102includes a seed or "template" layer 112 on the lead 102, a layer ofantiferromagnetic material 116 on the template layer 112, and a "fixed"ferromagnetic layer 118 formed on and exchange coupled with theunderlying antiferromagnetic layer 116. The ferromagnetic layer 118 iscalled the fixed layer because its magnetic moment or magnetizationdirection is prevented from rotation in the presence of applied magneticfields in the desired range of interest. The top electrode stack 130includes a "free" or "sensing" ferromagnetic layer 132 and a protectiveor capping layer 134 formed on the sensing layer 132. The sensingferromagnetic layer 132 is not exchange coupled to an antiferromagneticlayer, and its magnetization direction is thus free to rotate in thepresence of applied magnetic fields in the range of interest. Thesensing ferromagnetic layer 132 is fabricated so as to have its magneticmoment or magnetization direction (shown by arrow 133) orientedgenerally parallel to the ABS (the ABS is a plane parallel to the paperin FIG. 4) and generally perpendicular to the magnetization direction ofthe fixed ferromagnetic layer 118 in the absence of an applied magneticfield. The fixed ferromagnetic layer 118 in electrode stack 110 justbeneath the tunnel barrier layer 120 has its magnetization directionfixed by interfacial exchange coupling with the immediately underlyingantiferromagnetic layer 116, which also forms part of bottom electrodestack 110. The magnetization direction of the fixed ferromagnetic layer118 is oriented generally perpendicular to the ABS, i.e., out of or intothe paper in FIG. 4 (as shown by arrow tail 119).

Also shown in FIG. 4 is a biasing ferromagnetic layer 150 forlongitudinally biasing the magnetization of the sensing ferromagneticlayer 132, and an insulating layer 160 separating and isolating thebiasing layer 150 from the sensing ferromagnetic layer 132 and the otherlayers of the MTJ 100. The biasing ferromagnetic layer 150 is a hardmagnetic material, such as a CoPtCr alloy, that has its magnetic moment(shown by arrow 151) aligned in the same direction as the magneticmoment 133 of the sensing ferromagnetic layer 132 in the absence of anapplied magnetic field. The insulating layer 160, which is preferablyalumina (Al₂ O₃) or silica (SiO₂), has a thickness sufficient toelectrically isolate the biasing ferromagnetic layer 150 from the MTJ100 and the electrical leads 102, 104, but is still thin enough topermit magnetostatic coupling (shown by dashed arrow 153) with thesensing ferromagnetic layer 132. The product M*t (where M is themagnetic moment per unit area of the material in the ferromagnetic layerand t is the thickness of the ferromagnetic layer) of the biasingferromagnetic layer 150 must be greater than or equal to the M*t of thesensing ferromagnetic layer 132 to assure stable longitudinal biasing.Since the magnetic moment of Ni.sub.(100-x) --Fe.sub.(x) (x beingapproximately 19) that is typically used in the sensing ferromagneticlayer 132 is about twice that of the magnetic moment of a typical hardmagnetic material suitable for the biasing ferromagnetic layer 150, suchas Co₇₅ Pt₁₃ Cr₁₂, the thickness of the biasing ferromagnetic layer 150is at least approximately twice that of the sensing ferromagnetic layer132.

A sense current I is directed from first electrical lead 102perpendicularly through the antiferromagnetic layer 116, the fixedferromagnetic layer 118, the tunnel barrier layer 120, and the sensingferromagnetic layer 132 and then out through the second electrical lead104. As described previously, the amount of tunneling current throughthe tunnel barrier layer 120 is a function of the relative orientationsof the magnetizations of the fixed and sensing ferromagnetic layers 118,132 that are adjacent to and in contact with the tunnel barrier layer120. The magnetic field from the recorded data causes the magnetizationdirection of sensing ferromagnetic layer 132 to rotate away from thedirection 133, i.e., either into or out of the paper of FIG. 4. Thischanges the relative orientation of the magnetic moments of theferromagnetic layers 118, 132 and thus the amount of tunneling current,which is reflected as a change in electrical resistance of the MTJ 100.This change in resistance is detected by the disk drive electronics andprocessed into data read back from the disk. The sense current isprevented from reaching the biasing ferromagnetic layer 150 by theelectrical insulating layer 160, which also insulates the biasingferromagnetic layer 150 from the electrical leads 102, 104.

A representative set of materials for MTJ 100 (FIG. 4) will now bedescribed. All of the layers of the MTJ 100 are grown in the presence ofa magnetic field applied parallel to the surface of the substrate. Themagnetic field serves to orient the easy axis of all of theferromagnetic layers. A 5 nm Ta seed layer (not shown) is first formedon a 10-50 nm Au layer that serves as the electrical lead 102. The seedlayer is comprised of a material which encourages the (111) growth ofthe face-centered cubic (fcc) Ni₈₁ Fe₁₉ template layer 112. The templateferromagnetic layer 112 encourages the growth of the antiferromagneticlayer 116. Suitable seed layer materials include fcc metals, such as Cu,as well as Ta or a combination of layers, such as 3-5 nm Ta/3-5 nm Cu.The MTJ base electrode stack 110 comprises a stack of 4 nm Ni₈₁ Fe₁₉ /10nm Fe₅₀ Mn₅₀ /8 nm Ni₈₁ Fe₁₉ (layers 112, 116, 118, respectively) grownon the Ta seed layer on the 10-20 nm Au layer 102. The Au lead layer 102is formed on the alumina gap material G1 that serves as the substrate.Next, the tunnel barrier layer 120 is formed by depositing and thenplasma oxidizing a 0.5-2 nm Al layer. This creates the Al₂ O₃ insulatingtunnel barrier layer 120. The top electrode stack 130 is a 5 nmNi--Fe/10 nm Ta stack (layers 132, 134, respectively). The Ta layer 134serves as a protective capping layer. The top electrode stack 130 iscontacted by a 20 nm Au layer that serves as the electrical lead 104.

Note that since the current passes perpendicular to the layers in MTJ100, the resistance of the MTJ device will be largely dominated by thatof the tunnel barrier layer 120. Thus, the resistance per unit area ofthe conducting leads 102, 104 can be much higher than in conventional MRread heads in which the current flows parallel to the layers. Thus, theleads 102, 104 can be made thinner and/or narrower than in conventionalMR head structures, and/or can be made from intrinsically more resistivematerials, such as alloys or combinations of elements.

It is important that the layers in the bottom electrode stack 110 besmooth, and that the Al₂ O₃ tunnel barrier layer 120 be free of pinholeswhich would electrically short the junction. For example, growth bysputtering techniques known to produce good giant magnetoresistanceeffects in metallic multilayer stacks is sufficient.

An alternative sensing ferromagnetic layer 132 may be comprised of athin Co or Co.sub.(100-x) Fe.sub.(x) or Ni.sub.(100-x) Fe_(x) (x isapproximately 60) layer at the interface between the sensingferromagnetic layer 132 and the tunnel barrier layer 120, with the bulkof layer 132 being a low magnetostriction material, such asNi.sub.(100-x) Fe_(x) (x is approximately 19). The net magnetostrictionof this type of sensing layer with a thin Co or Co.sub.(100-x)Fe.sub.(x) or Ni.sub.(100-x) Fe_(x) (x is approximately 60) interfacelayer is arranged to have a value close to zero by slight variations ofthe composition of the bulk of layer 132. An alternative fixedferromagnetic layer 118 may be comprised largely of a bulkNi.sub.(100-x) Fe.sub.(x) layer with a thin layer of a Co orCo.sub.(100-x) Fe.sub.(x) or Ni.sub.(100-x) Fe_(x) (x is approximately60) layer at the interface with the tunnel barrier layer 120. Thelargest signal is obtained with Co or with the highest polarizationNi.sub.(100-x) Fe_(x) (x is approximately 60) or Co.sub.(100-x)Fe.sub.(x) alloy (x is approximately 70). The interface layer isoptimally about 1-2 nm thick. The net magnetostriction of the combinedlayer is arranged to be close to zero by small variations of thecomposition. If the bulk of layer 118 is Ni--Fe, then the composition isNi₈₁ Fe₁₉, the composition for which bulk Ni--Fe has zeromagnetostriction.

The Fe--Mn antiferromagnetic layer 116 may be replaced with a Ni--Mnlayer or other suitable antiferromagnetic layer which exchange biasesthe ferromagnetic material in the fixed layer 118 and which has aresistance which is substantially less than that of the Al₂ O₃ barrierlayer 120. In addition, while in the preferred embodiment the fixedferromagnetic layer has its magnetic moment fixed by interfacialexchange coupling with an antiferromagnetic layer, the fixedferromagnetic layer may be formed of a magnetically "hard" highcoercivity material, thereby avoiding the need for an antiferromagneticlayer. The hard fixed ferromagnetic layer may thus be formed from avariety of ferromagnetic materials, such as alloys of Co and one or moreother elements, including Co--Pt--Cr alloys, Co--Cr--Ta alloys, Co--Cralloys, Co--Sm alloys, Co--Re alloys, Co--Ru alloys, and Co--Ni--Xalloys (X=Pt, Pd, or Cr), as well as a variety of quaternary alloys,such as Co--Ni--Cr--Pt and Co--Pt--Cr--B.

While the MTJ device described and shown in FIG. 4 has the fixedferromagnetic layer on the bottom of MTJ 100, the device can also beformed by depositing the sensing ferromagnetic layer first, followed bythe tunnel barrier layer, the fixed ferromagnetic layer, and theantiferromagnetic layer. Such an MTJ device would then have the layersessentially inverted from the MTJ 100 shown in FIG. 4.

Referring now to FIG. 5, the rear flux guided MTJ MR head of the presentinvention is illustrated in a section view perpendicular to the view ofFIG. 4 and with the sensing surface 200 or ABS to the right. For ease ofexplanation, the biasing ferromagnetic layer 150 is not shown and in theMTJ 100 only the ferromagnetic layers, the antiferromagnetic layer andthe tunnel barrier layer are shown. The sensing ferromagnetic layer 132has a sensing edge 202 that is substantially coplanar with the sensingsurface 200 or ABS and a back edge 203. The fixed ferromagnetic layer118 has a front edge 206 that is substantially coplanar with the sensingsurface 200 or ABS and a back edge 208. The antiferromagnetic layer 116has edges contiguous with the edges of the fixed ferromagnetic layer118. The tunnel barrier layer 120 has a front edge 210 that is alsosubstantially coplanar with the sensing surface 200 or ABS and which isessentially coplanar with front edge 206 of the fixed ferromagneticlayer 118, and a back edge 212. The back edge of the sensingferromagnetic layer 203 extends beyond the back edge of either thetunnel barrier layer 212 or the fixed ferromagnetic layer 208, whicheverback edge is closer to the sensing surface 200. The lead 102 is formedon the G1 gap layer and the G2 gap layer separates the lead 104 frommagnetic shield S2. The material of G1 and G2 and in the region behindback edges 203, 208 and 212 is an electrically insulating material,preferably alumina. The sensing surface 200 or ABS may have a protectiveovercoat formed on it, such as a thin layer of amorphous diamond-likecarbon, as is known in the art to protect the head during contact withthe disk.

As shown in FIG. 3 a MR sensor, such as sensor 40, is placed betweenpermeable magnetic shields S1 and S2. When the MTJ read head is locatedin this region, as shown in FIG. 5, the magnetic flux to be detectedenters the front edge 202 of the free ferromagnetic layer 132 at theair-bearing surface 200 and decays towards the read edge 203 of thislayer. The flux is constrained to be zero at the rear edge 203 of thislayer. Some of the incident magnetic flux leaks to the magnetic shieldsS1 and S2. This leakage is determined by the shield--shield gap width g,and the permeability μ and thickness t of the free ferromagnetic layer132. The magnetic flux thus decays from the front edge of the freeferromagnetic layer with a characteristic length λ of (μtg/2)⁰.5.Typical parameters for a 5 Gbit/in² sensor are g=200 nm, t=5 nm andμ=1000. This results in a decay length λ of 0.7 micron.

In the earlier U.S. Pat. No. 5,390,061 describing the use of an MTJdevice for a magnetic recording read head the rear edge of the freeferromagnetic layer was contiguous with that of the fixed ferromagneticlayer or closer to the ABS than the rear edge of the fixed ferromagneticlayer. Thus for high density recording applications in which the heightof the sensor is comparable to or less than λ (for example, for a 5Gbit/in² sensor the sensor height is 400 nm) the magnetic flux isconstrained to be zero in the active region of the sensor (the regionbetween the ABS 200 and the rear edge 212 of the tunnel barrier layer120 or the rear edge 208 of the fixed ferromagnetic layer 118, whicheveredge is closest to the ABS 200). By extending the free ferromagneticlayer 132 beyond the active region of the sensor by the addition of aflux guide at the rear edge of the free ferromagnetic layer then theflux is constrained to be zero only at the rear edge of the rear fluxguide. Thus the amount of magnetic flux in the active region of thesensor is increased beyond that in the absence of such a rear fluxguide. Consequently, the output signal of the MTJ sensor with the rearflux guide is enhanced by the amount of extra flux in the active regionof the sensor.

Because current flows perpendicular to the tunnel junction and thus tothe portion of the flux guide that forms part of the tunnel junction, nocurrent is shunted away by the portion of the free ferromagnetic layerthat extends beyond the tunnel junction. While in the preferredembodiment of FIG. 5 the back edges 212, 208 of the tunnel barrier layer120 and the fixed ferromagnetic layer 118, respectively, are coplanar,they need not be so, provided the back edge 203 of the freeferromagnetic layer 132 is farther from the sensing surface 200 thanwhichever of back edges 212, 208 is closer to the sensing surface 200.This is because the current flow perpendicular through the tunnelbarrier layer 120 is defined by whichever back edge 212, 208 is closerto the sensing surface 200. Thus the back edge 203 of the sensingferromagnetic layer 132 is located farther than the back edge 212 of thetunnel barrier layer 120 if the back edge 212 is closer to the sensingsurface 200 than the back edge 208 of the fixed ferromagnetic layer 118.Similarly, the back edge 203 of the sensing ferromagnetic layer 132 islocated farther than the back edge 208 of the fixed ferromagnetic layer118 if the back edge 208 is closer to the sensing surface 200 than theback edge 212 of the tunnel barrier layer 120.

While in the preferred embodiment shown and described with respect toFIG. 5 the first lead 102 is shown as having its back edge extendingbeyond the back edges of the antiferromagnetic layer 116 and fixedferromagnetic layer 118, the first lead 102 may have its back edgesubstantially coplanar with the back edges of the antiferromagneticlayer 116 and fixed ferromagnetic layer 118. Also while the second lead104 is shown as having its back edge coplanar with the back edge 203 ofthe free ferromagnetic layer 132, the second lead 104 may have its backedge extending beyond the back edge 203 of the free ferromagnetic layer132. Similarly the second lead 104 may have its back edge closer to theABS 200 than the back edge 203 of the free ferromagnetic layer 132providing that its back edge is further from the ABS 200 than the rearedge of the active region of the sensor.

In an alternative embodiment the substrate onto which the first lead 102is formed is the first magnetic shield S1 and the second magnetic shieldS2 is formed on the second lead 104. The shields S1 and S2 are formed ofNi--Fe alloys or Ni--Fe--Co alloys and are electrically conducting. Inthis embodiment an electrically conductive path is thus provided throughthe shield S1 to first lead 102, perpendicularly through the tunneljunction to the second lead 104 and the second shield S2. Thisembodiment eliminates the need for insulative gap layers G1, G2,although insulative material is still required at the back of the tunneljunction, as shown in FIG. 5.

Process for Fabricating the Recessed MTJ MR Read Head

For AMR or spin valve sensors where the sense current flows parallel tothe layers of the sensor, adjoining the flux guide to the sensor isdifficult since the flux guide and the sensor must be electricallyisolated so as not to shunt the current from the sensor. Yet the fluxguide and the sensor must be magnetically coupled. This can beaccomplished by insulating the sensor with a dielectric layer and thenaligning a flux guide shape to the sensor. Efficient magnetic couplingto the sensor requires alignments of 0.1 μm accuracy and dielectricthicknesses of 5 nm to 10 nm, both of which are difficult to accomplishwith present day technologies used for MR head fabrication. However, ifa magnetic tunnel junction sensor is used, where current flowsperpendicularly to the layers of the sensor, then the flux guide neednot be electrically isolated from the sensing layer. The rear flux guidecan be a continuous extension of the free ferromagnetic layer, asillustrated in the preferred embodiment in FIG. 5.

Referring to FIG. 6, the process for forming the rear flux guided MTJ MRread head will be described. Two lithographic patterning steps arerequired. One defines the back edge of the fixed ferromagnetic layer 118and one defines the back edge of the free ferromagnetic layer 132. TheMTJ MR read head is fabricated on a layer of insulator, typically thealumina G1 layer, as shown in FIG. 5, but can also be fabricateddirectly on the bottom magnetic shield layer S1.

The process begins, as shown in FIG. 6A, by depositing the material forlead layer 102, an antiferromagnetic layer 116, the fixed ferromagneticlayer 118, and a material, such as aluminum, that will ultimately beoxidized to form the tunnel barrier layer 120. The lead material can bea variety of conducting materials, such as Ta, Al, Cu, Au, W and Pt witha typical thickness in the range of 100 to 500 Å. The antiferromagneticlayer 116 can be selected from a variety of well-known materials, suchas Fe--Mn, Ni--Mn, Pt--Mn, Ir--Mn, Pd--Mn and Cr--Al. The typicalthickness for the antiferromagnetic layer 116 is in the range of 70 to300 Å. The fixed ferromagnetic layer 118 is preferably a Ni--Fe alloy ora bilayer of Ni--Fe alloy and a thin film of Co. Typical thicknesses forthe Ni--Fe alloy layer are 20 to 100 Å and typical thicknesses for theCo layer are 2 to 20 Å. The thickness of the aluminum for the tunnelbarrier oxide layer 120 is typically in the range of 5 to 20 Å.

After deposition of these layers, which is usually either by ion beamdeposition or rf or dc magnetron sputtering, the layers are patternedusing resist 230 to define the desired shape shown in FIG. 6B, which isa top view of FIG. 6A. Ion milling then removes material not protectedby resist 230, as shown in FIG. 6C. The lead layer 102,antiferromagnetic layer 116, fixed ferromagnetic layer 118 and tunnelbarrier layer are now formed on layer G1 with the shape shown in FIG.6D. The resist layer 230 is typically a bilayer resist with an undercut.After the ion milling step of FIG. 6C a layer of insulator 232,typically alumina or SiO₂, is deposited by ion beam or RF sputtering toseal the edges of the pattern, after which the resist layer 230 islifted off, resulting in the structure shown in FIGS. 6E-6F. In thisfirst lithographic patterning step the rear edge 208 of the fixedferromagnetic layer 118 is set as a reference point.

After patterning to form the structure of FIGS. 6E-6F, the aluminum inwhat will become the tunnel barrier layer 120 is plasma oxidized at anoxygen pressure of 100 mTorr and a power density of 25 W/cm² for 30-240seconds. This forms the insulating tunnel barrier layer 120 of alumina.

Next, as shown in FIGS. 6G-6H, the free ferromagnetic layer 132 and thelead layer 104 are deposited. The free ferromagnetic layer 132 istypically a Ni--Fe alloy or a bilayer of Co and a Ni--Fe alloy, with athickness from 10 to 200 Å for the Ni--Fe alloy and a thickness of 2 to20 Å for the Co. The lead 104 is formed of similar materials andthicknesses as described for the lead 102.

After deposition of layers 132, 104, which are done by either ion beamdeposition or RF or dc magnetron sputtering, the free ferromagneticlayer 132 and lead layer 104 are patterned using resist 240 to definethe desired shape, as shown in FIGS. 6I-6J. The resist layer 240 istypically a bilayer resist with an undercut. Ion milling then removesmaterial not protected by resist 240, as shown in FIGS. 6K-6L. After theion milling step of FIG. 6K a layer of insulator 242, typically aluminaor SiO₂, is deposited by ion beam or RF sputtering to seal the edges ofthe pattern, after which the resist layer 240 is lifted off, resultingin the structure shown in FIGS. 6M-6N. An important feature defined inthis second lithographic patterning is the width of the freeferromagnetic layer 132, i.e., the width w that will be exposed at theABS. This step also defines the back edge 203 so that the freeferromagnetic layer 132 extends from the ABS and over the front edge 210and back edge 212 of the tunnel barrier layer 120 and terminates beyondthe back edge 212. As described previously, this assists in propagatingflux efficiently across the entire active tunnel junction area definedby the back edges of the tunnel barrier layer 120 and fixedferromagnetic layer 118 and the ABS 200.

The above process can also be adapted to provide longitudinal biasing orstabilization for the free ferromagnetic layer 132 that also serves asthe flux guide, as described previously with respect to the biasingferromagnetic layer 150 shown in FIG. 4. Specifically, the steps shownin FIGS. 6K-N are modified so that instead of depositing the aluminalayer 242 and then lifting off the resist 240, a sequential depositionof alumina, hard biasing ferromagnetic material, and additional aluminais performed and then liftoff is done. The resulting structure is shownin FIG. 7, which is a view of sensing surface 200. FIG. 7 shows thesensing ferromagnetic layer 132 and second lead 104 with their frontedges exposed at the sensing surface 200. Also shown are the exposededges of the biasing ferromagnetic layers 150. The regions between thehard biasing ferromagnetic layers 150 and the sensing ferromagneticlayer 132, the first lead 102 and second lead 104 are formed ofinsulative material, such as alumina. The typical alumina thicknessesare in the 100 to 500 Å range and the hard biasing ferromagneticmaterial is usually a Co--Pt alloy with a thickness adjusted to provide1 to 3 times the moment of the free ferromagnetic layer 132. The firstalumina insulation covers the edges of the sensing ferromagnetic shapeand the second alumina insulation covers the top surface of the hardbiasing ferromagnetic material. After the liftoff, a final patterningstep is used to remove unwanted regions of hard biasing ferromagneticmaterial.

The total thickness of the leads, free and fixed ferromagnetic layer,tunnel oxide layer, and antiferromagnetic layer are limited by the totalseparation between the shields S1 and S2. For a 5 Gbit/in² sensor thisnumber ranges from 1000 to 2000 Å. It is advantageous to have the freeferromagnetic layer 132 centered in this gap between the two shields.This can be accomplished by adjusting the ratio of thicknesses of theleads 104, 102.

After the lead 104 and free ferromagnetic layer 132 have been patternedand the MTJ MR head structure is essentially complete but for thelapping step to form the ABS 200, it is still necessary to align themagnetization direction (magnetic moment) of the fixed ferromagneticlayer 118 in the proper direction. If Fe--Mn is used as theantiferromagnetic layer 116 for exchange coupling with the fixedferromagnetic layer 118 it is antiferromagnetic as deposited. However,its magnetization must be realigned so that it can exchange couple thefixed ferromagnetic layer 118 in the proper orientation. The structureis placed in an annealing oven and the temperature is raised toapproximately 180° C., which is greater than the blocking temperature ofFe--Mn. At this temperature, the Fe--Mn layer no longer gives rise to anexchange anisotropy with the fixed ferromagnetic layer 118. An exchangeanisotropy of the ferromagnetic layer 118 is developed by cooling thepair of layers 116, 118 in a magnetic field. The orientation of themagnetization of the fixed ferromagnetic layer 118 will be along thedirection of the applied magnetic field. The applied magnetic field inthe annealing oven thus causes the moment of the fixed ferromagneticlayer 118 to be fixed along the required direction perpendicular to theABS, as shown by the arrow 119 in FIG. 4. This is a result of coolingthe Fe--Mn layer in the presence of the ferromagnetic layer 118,magnetized by the applied magnetic field, in the required direction.Thus, at temperatures below the blocking temperature of Fe--Mn, in thepresence of an applied magnetic field from the recorded medium, themagnetization of the fixed ferromagnetic layer 118 will notsubstantially rotate.

As an alternative structure to the preferred embodiment shown in FIG. 5,the lead 104 and the free ferromagnetic layer 132 can be formed first onthe G1 substrate, with the fixed ferromagnetic layer 118,antiferromagnetic layer 116 and lead 102 being on the "top" of the MTJduring the fabrication process.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

What is claimed is:
 1. A magnetic tunnel junction magnetoresistive readhead for sensing data magnetically recorded on a medium when connectedto sense circuitry, the head having a substantially planar sensingsurface that is aligned generally parallel to the surface of the mediumwhen the magnetically recorded data is being sensed, the headcomprising:a substrate having an edge forming part of the sensingsurface; a first electrically conductive lead formed on the substrate; afixed ferromagnetic layer formed on the first lead and having a frontedge substantially coplanar with the sensing surface and a back edgelocated farther than the front edge from the sensing surface, themagnetization direction of the fixed ferromagnetic layer being fixedalong a preferred direction so as to be substantially prevented fromrotation in the presence of an applied magnetic field from the medium; asensing ferromagnetic layer having a sensing edge substantially coplanarwith the sensing surface and a back edge, the magnetization direction ofthe sensing ferromagnetic layer being oriented in a direction generallyperpendicular to the magnetization direction of the fixed ferromagneticlayer in the absence of an applied magnetic field and being free torotate in the presence of an applied magnetic field from the medium; aninsulating tunnel barrier layer located between and in contact with thefixed and sensing ferromagnetic layers for permitting tunneling currentin a direction generally perpendicular to the fixed and sensingferromagnetic layers, the tunnel barrier layer having a front edgesubstantially coplanar with the sensing surface and a back edge, theback edge being located farther than the front edge from the sensingsurface; a second electrically conductive lead formed on the sensingferromagnetic layer; and wherein the back edge of the sensingferromagnetic layer is located farther than the back edge of the tunnelbarrier layer from the sensing surface if the back edge of the tunnelbarrier layer is closer to the sensing surface than the back edge of thefixed ferromagnetic layer, and farther from the sensing surface than theback edge of the fixed ferromagnetic layer if the back edge of the fixedferromagnetic layer is closer to the sensing surface than the back edgeof the tunnel barrier layer.
 2. The head according to claim 1 whereinthe front edge of the tunnel barrier layer and the front edge of thefixed ferromagnetic layer are substantially coplanar.
 3. The headaccording to claim 1 wherein the back edge of the tunnel barrier layerand the back edge of the fixed ferromagnetic layer are substantiallycoplanar and wherein the back edge of the sensing ferromagnetic layer islocated farther than the coplanar back edges of the tunnel barrier layerand fixed ferromagnetic layer from the sensing surface.
 4. The headaccording to claim 1 further comprising an antiferromagnetic layer incontact with the fixed ferromagnetic layer for fixing the magnetizationdirection of the fixed ferromagnetic layer by interfacial exchangecoupling, the antiferromagnetic layer having a front edge substantiallycoplanar with the sensing surface.
 5. The head according to claim 1wherein the back edges of the antiferromagnetic layer, the fixedferromagnetic layer and the tunnel barrier layer are substantiallycoplanar.
 6. The head according to claim 1 wherein the first electricallead is formed directly on the substrate and wherein theantiferromagnetic layer is located between the first electrical lead andthe fixed ferromagnetic layer, the fixed ferromagnetic layer beingformed directly on and in contact with the antiferromagnetic layer,whereby the magnetization direction of the fixed ferromagnetic layer isfixed by interfacial exchange coupling with the antiferromagnetic layer.7. The head according to claim 1 wherein the magnetization direction ofthe sensing ferromagnetic layer is generally parallel to the sensingsurface in the absence of an applied magnetic field.
 8. The headaccording to claim 1 further comprising:a biasing ferromagnetic layerfor longitudinally biasing the magnetization direction of the sensingferromagnetic layer in a direction generally perpendicular to themagnetization direction of the fixed ferromagnetic layer in the absenceof an applied magnetic field; and an electrically insulating layerlocated between the biasing and sensing ferromagnetic layers forelectrically isolating the biasing layer from the sensing layer; andwherein the electrical leads are electrically isolated from the biasinglayer by the insulating layer, whereby when a sense current is passedbetween the fixed ferromagnetic layer and the sensing ferromagneticlayer it passes generally perpendicularly through the tunnel barrierlayer without passing into the biasing layer.
 9. The head according toclaim 1 wherein the read head is part of an integrated read/write headof the type wherein the read head is shielded from the write head andwherein said substrate is a first shield for the read head.
 10. The headaccording to claim 9 further comprising a layer of electricallyinsulative gap material formed on the first shield and wherein the firstelectrical lead is formed on the layer of gap material.
 11. The headaccording to claim 1 further comprising a second substrate, wherein thefirst lead, the sensing ferromagnetic layer, the tunnel barrier layer,and the second lead form a stack of layers located between the first andsecond substrates, and further comprising insulative material locatedbetween said stack and the first and second substrates.
 12. The headaccording to claim 11 wherein the read head is part of an integratedread/write head of the type wherein the read head is magneticallyshielded and wherein the second substrate is a second shield separatingthe read head from the write head.
 13. The head according to claim 1further comprising sense circuitry connected to the first and secondleads.
 14. The head according to claim 1 wherein the substrate is afirst electrically conducting magnetic shield and wherein the first leadis formed on the first shield, whereby an electrically conductive pathis provided between the first shield and the first lead.
 15. The headaccording to claim 14 further comprising a second electricallyconducting magnetic shield formed on the second lead, whereby anelectrically conductive path is provided from the first shield to thefirst lead and through the tunnel barrier layer to the second lead andthe second shield.
 16. The head according to claim 1 wherein the head isthe type for sensing data from a magnetic recording disk and furthercomprising an air-bearing slider having an air-bearing surface (ABS)facing the surface of the disk when data from the disk is being read bythe head and a trailing end surface generally perpendicular to the ABS,and wherein the slider trailing end surface is the substrate on whichthe first electrical lead is formed and the slider ABS is the sensingsurface of the head.
 17. A magnetic tunnel junction magnetoresistiveread head for sensing data magnetically recorded on a medium whenconnected to sense circuitry, the head having a substantially planarsensing surface that is aligned generally parallel to the surface of themedium when the magnetically recorded data is being sensed, the headcomprising:a substrate having an edge forming part of the sensingsurface; a first electrically conductive lead formed on the substrate; afixed ferromagnetic layer formed on the first lead and having a frontedge substantially coplanar with the sensing surface and a back edgelocated farther than the front edge from the sensing surface; anantiferromagnetic layer in contact with the fixed ferromagnetic layerfor fixing the magnetization direction of the fixed ferromagnetic layerby interfacial exchange coupling along a preferred direction so it issubstantially prevented from rotation in the presence of an appliedmagnetic field from the medium, the antiferromagnetic layer having afront edge substantially coplanar with the sensing surface; a sensingferromagnetic layer having a sensing edge substantially coplanar withthe sensing surface and a back edge, the magnetization direction of thesensing ferromagnetic layer being oriented in a direction generallyperpendicular to the magnetization direction of the fixed ferromagneticlayer and generally parallel to the sensing surface in the absence of anapplied magnetic field and being free to rotate in the presence of anapplied magnetic field from the medium; an insulating tunnel barrierlayer located between and in contact with the fixed and sensingferromagnetic layers for permitting tunneling current in a directiongenerally perpendicular to the fixed and sensing ferromagnetic layers,the tunnel barrier layer having a front edge substantially coplanar withthe sensing surface and a back edge located farther than the tunnelbarrier layer front edge from the sensing surface; a second electricallyconductive lead formed on the sensing ferromagnetic layer; and whereinthe back edge of the sensing ferromagnetic layer is located farther thanthe back edge of the tunnel barrier layer from the sensing surface ifthe back edge of the tunnel barrier layer is closer to the sensingsurface than the back edge of the fixed ferromagnetic layer, and fartherfrom the sensing surface than the back edge of the fixed ferromagneticlayer if the back edge of the fixed ferromagnetic layer is closer to thesensing surface than the back edge of the tunnel barrier layer.
 18. Thehead according to claim 17 wherein the back edges of theantiferromagnetic layer, the fixed ferromagnetic layer and the tunnelbarrier layer are substantially coplanar.
 19. The head according toclaim 17 wherein the back edge of the tunnel barrier layer and the backedge of the fixed ferromagnetic layer are substantially coplanar andwherein the back edge of the sensing ferromagnetic layer is locatedfarther than the coplanar back edges of the tunnel barrier layer andfixed ferromagnetic layer from the sensing surface.
 20. The headaccording to claim 17 wherein the first electrical lead is formeddirectly on the substrate and wherein the antiferromagnetic layer islocated between the first electrical lead and the fixed ferromagneticlayer, the fixed ferromagnetic layer being formed directly on and incontact with the antiferromagnetic layer.
 21. The head according toclaim 17 further comprising:a biasing ferromagnetic layer forlongitudinally biasing the magnetization direction of the sensingferromagnetic layer in a direction generally perpendicular to themagnetization direction of the fixed ferromagnetic layer in the absenceof an applied magnetic field; and an electrically insulating layerlocated between the biasing and sensing ferromagnetic layers forelectrically isolating the biasing layer from the sensing layer; andwherein the electrical leads are electrically isolated from the biasinglayer by the insulating layer, whereby when a sense current is passedbetween the fixed ferromagnetic layer and the sensing ferromagneticlayer it passes generally perpendicularly through the tunnel barrierlayer without passing into the biasing layer.
 22. The head according toclaim 17 wherein the read head is part of an integrated read/write headof the type wherein the read head is magnetically shielded and whereinsaid substrate is a first shield for the read head.
 23. The headaccording to claim 22 further comprising a layer of electricallyinsulative gap material formed on the first shield and wherein the firstelectrical lead is formed on the layer of gap material.
 24. The headaccording to claim 17 further comprising sense circuitry connected tothe first and second leads.
 25. The head according to claim 17 whereinthe substrate is a first electrically conducting magnetic shield andwherein the first lead is formed on the first shield, and furthercomprising a second electrically conducting magnetic shield formed onthe second lead, whereby an electrically conductive path is providedfrom the first shield to the first lead and through the tunnel barrierlayer to the second lead and the second shield.
 26. The head accordingto claim 17 wherein the head is the type for sensing data from amagnetic recording disk and further comprising an air-bearing sliderhaving an air-bearing surface (ABS) facing the surface of the disk whendata from the disk is being read by the head and a trailing end surfacegenerally perpendicular to the ABS, and wherein the slider trailing endsurface is the substrate on which the first electrical lead is formedand the slider ABS is the sensing surface of the head.